No one can be sure how morphs will interact until it has been done, and I don't remember hearing about this one.

However, as a general rule, white markings on animals are because there is something preventing any pigmentation on that part of the animal. So, even when combined with another morph, white "should" stay white.

However, as you pointed out, super mojaves aren't quite pure white. So, there is at least some chance the spider pattern would show through, at least on the head. I've also heard that some BELs show faint hints of pattern, like on a snow but much more faint. So you might be able to see the spider pattern on at least some BELs, just very faintly.